It has been proposed to estimate the flatness of a surface on a test object by visually observing reflection through a dye liquid film or layer placed between the test surface and a flat master surface. For example, U.S. Pat. No. 2,695,544 discloses a system consisting of, in order, a pane of glass, a dye layer, and the test object. Light is directed through the pane of glass and into the dye layer. The operator then visually observes the light reflected by the test object surface back through the dye layer and the glass pane. Since the light energy is attenuated as a function of distance traveled through the dye layer, departure of the reflected light from uniform intensity across the image generally indicates a corresponding departure of the test object surface from flatness or parallelism with the surface of the glass pane. This method is limited to a subjective and qualitative estimate of the flatness of the test object. This method is also limited by the visual acuity of the operator which will, of course, vary from operator to operator. This method cannot account for differences in reflectivity of the test object across its surface or for differences in the illumination or for other artifacts. This method is suitable for use only in relatively less-demanding quality control applications where parts are either accepted or rejected depending on their qualitative deviation from a prescribed geometry. This method is generally not suitable for use in the operation, control, and/or modification of a manufacturing process wherein the parts are produced. This method is generally not useful in quality control or other operations where it is necessary to quantitatively determine the deviations of the test object from a prescribed geometry.
It is desirable, therefore, to provide a system and a method for gauging the deviations of a test object from a predetermined nominal profile geometry with enhanced and improved capabilities for precise quantitative measurement of surface deviations over the entire surface of the test object. It is also desirable to provide a system and a method for gauging the deviations of a test object from a predetermined nominal profile geometry with improved measurement resolution that is adapted for use in conjunction with test objects having a wide variety of geometries and optical characteristics. It is also desirable to provide such a system and method that includes a calibration means for correcting optically generated errors and for compensating against effects of background radiation, variations in illumination intensity, and variations in test object reflectivity. It is also desirable to provide such a system and method that allows for improved control of measurement resolution, and that is readily adapted for automation. The present invention provides an improved system and method for gauging the deviations of a specular surfaced test object from a predetermined nominal profile geometry which achieves these just described objectives and criteria.